Gas generator fuel composition

ABSTRACT

The invention relates to solid gas generator fuel compositions (gas producing mixtures), comprising: (A) nitroguanidine or nitroguanidine stabilised with 0.1 to 0.5% nitroguanidinium hydrogen sulphate and nitroguanidinium nitrate as fuel; (B) an oxidising agent selected from the group comprising alkali and earth alkali nitrates, chlorates and perchlorates, ammonium nitrate and perchlorate, copper compounds with oxidising properties and mixtures thereof; (C) a stabiliser selected from the group comprising hydrophobic SiO 2 , inorganic and organic acids and mixtures thereof; and optionally (D) a combustion stabiliser or moderator and ash former or trap and mixtures thereof. The above gas generator fuel compositions comprise an improved shelf-life under hot storage conditions at 110° C.

[0001] The present invention relates to solid propellants for gas generators (gas-generating mixtures), wherein said propellants are mainly intended for use in propelling charges for gas generators used in airbags or seat-belt pretentioning devices, wherein the propellant for gas generators has a very good long-term thermal stability.

[0002] An airbag essentially comprises a housing of the gas generator filled with the propelling charges for gas generators, generally in tablet form, and an initial detonator (squib) for detonating the propelling charge for gas generators, and also a gas bag. Suitable detonators are disclosed, for example in U.S. Pat. No. 4,931,111. The gas bag, which is initially folded into a small bag, is filled, after the initial detonation, with the gases produced in the burn-up of the propelling charge for gas generators and reaches its full volume in a time period of about 10-50 ms. The exit of hot sparks, molten material and solids from the gas generator into the gas bag has to be largely prevented, since it could result in the destruction of the gas bag or in injury to the vehicle passengers. This is achieved by binding and filtering of the slag formed by burning up the propelling charge for gas generators.

[0003] Propelling charges for gas generators to be used in airbags on the basis of sodium azide are well-known. However, the use of the highly toxic sodium azide requires a work intensive and costly process for the preparation of propelling charges for gas generators. Furthermore, the worldwide ever increasing number of non-burned-up propelling charges for gas generators in used vehicles leads to a disposal and security problem.

[0004] Therefore, in recent years there have been efforts to find suitable substitute propellants for gas generators which do not contain sodium azide or other toxic components.

[0005] DE-A-44 35 790 of the present applicant discloses propellants for gas generators on the basis of guanidine compounds on suitable carriers, which essentially exhibit an improved burn-up behaviour and an improved building of the slag. The propellant for gas generators disclosed in DE-A-44 35 790 comprises (A) at least one carbonate, hydrogen carbonate or a nitrate of guanidine, aminoguanidine, diaminoguanidine or triaminoguanidine in an amount of about 20-55 percent by weight based on the total amount of the components (A) and (B), (B) at least one alkaline or alkaline earth nitrate or ammonium nitrate as an oxidizing agent in an amount of about 80-45 percent by weight based on the total amount of components (A) and (B), and to moderate burn-up and to improve the formation of slags in an amount of 5-45 percent by weight based on the total amount of the components (A) and (B), (Cl) at least one carrier selected from silicon dioxide, alkaline, alkaline earth or alumosilicates and/or (C2) at least one oxygen supplying carrier selected from iron(III) oxide, cobalt oxides, manganese dioxide and cupric oxide. However, this document is not concerned with the technical problem of providing long-term stability of propellants for gas generators under elevated temperatures. With respect to the stability DE-A-44 35 790 refers to the Holland test according to which the propellant for gas generators is heated for 72 hours at 105° C. The Holland test is a test method established in the year 1927 for determining the chemical resistance of propellants. According to this test method the loss of weight is determined after heating at 105° C. for 72 hours (for polybasic propellants) and 110° C. (for monobasic propellants), respectively. The loss of weight minus the loss of weight occurring within the first eight hours may be 2% at maximum (see J. Köhler and R. Meyer, “Explosives”, 9th revised and extended edition 1998, Wiley-VCH, page 170).

[0006] DE-A-19812372, also of the present applicant, discloses propellants for gas generators, comprising:

[0007] (A) at least one fuel selected from the group comprising guanidine nitrate, dicyanamide, ammonium dicyanamide, sodium dicyanamide, copper dicyanamide, tin dicyanamide, calcium dicyanamide, guanidine dicyanamide, ammonium guanidine bicarbonate, ammonium guanidine nitrate, triamino guanidine nitrate, nitroguanidine, dicyan diamide, azodicarbonamide as well as tetrazole, 5-aminotetrazole, 5-nitro-1,2,4-triazole-3-one, salts and mixtures thereof,

[0008] (B) at least one alkali metal nitrate or alkaline earth metal nitrate or ammonium nitrate, -chlorate or -perchlorate,

[0009] (C) at least one essentially chemically-inert slag trap with a high fusion point, selected from the group comprising Al₂O₃, TiO₂ and ZrO₂ in highly dispersed form or mixtures thereof, and optionally

[0010] (D) at least one slag former, selected from alkali metal and alkaline earth metal carbonates, alkali metal and alkaline earth metal oxides, silicates, aluminates and aluminumsilicates, iron(III) oxide and silicon nitride which provides during burn-up nitrogen and silicon dioxide for the further reaction, and optionally (E) at least one binder being soluble in water at room temperature.

[0011] The essentially chemically-inert slag trap with high fusion point in highly dispersed form, i.e. these compounds are prepared by way of flame hydrolysis, functions as an internal filter and, thus, substantially prevents the formation and exit of dust type slag portions from the housing of the gas generator. A part of the highly dispersed slag traps can serve as a carrier for catalyst metals. Thus, this document is not concerned with the long-term stability of propellants for gas generators und heat aging conditions.

[0012] In view of the ever increasing number of different airbag systems in motor vehicles, such as driver-side airbag, passenger-side airbag, side-airbag and torax-airbag, and in view of the increasing life-time of motor vehicles due to the technical development, the automobile industry recently intensified the requirements with respect to the stability of propellants for gas generators. In this context experiments by the present inventors revealed that conventional propellants for gas generators containing nitroguanidine as fuel do not show a sufficient stability.

[0013] As far as evident the technical problem of long-term stabilisation of propellants for gas generators under heat aging (heat storage) has hitherto not been taken into consideration sufficiently in the prior art.

[0014] Accordingly, the object underlying the present invention is to provide propellants for gas generators which comply with the more strict requirements more and more requested by the automobile industry with respect to the stability under heat aging over at least 400 hours at a temperature of 110° C. under conservation of functionality.

[0015] This object underlying the present invention is solved by a propellant for gas generators, comprising:

[0016] (A) nitroguanidine (NIGU, NQ) as a fuel,

[0017] (B) an oxidizing agent selected from the group consisting of alkali metal and alkaline earth metal nitrates, -chlorates and -perchlorates, ammonium nitrate and -perchlorate, oxidizing copper compounds and mixtures thereof,

[0018] (C) a stabilizer selected from the group consisting of inorganic and organic acids and mixtures thereof, optionally

[0019] (D) a burn-up stabilizer or moderator, respectively, and slag former/slag trap, respectively, and mixtures thereof, and optionally

[0020] (E) at least one binder.

[0021] Accordingly, the present invention provides a propellant for gas generators which withstands a heat aging at 110° C. for at least 400 hours and, thus, complies with the increasingly strict requirements of the automobile industry with respect to propellants for gas generators or used in propelling charges for gas generators used in airbag systems.

[0022] It has been found that it is surprisingly possible to formulate propellants for gas generators based on nitroguanidine as a fuel and on the above-referenced oxidizing agents and oxidizing agent mixtures, respectively, in combination with one or more of the above-mentioned stabilizers which propellants for gas generators show under a heat aging at 110° C. for 400 hours, preferably 1000 hours and in particular 3000 hours, a loss of weight of less than 1%, preferably less than 0.5% and in particular less than 0.2% under conservation of the functionality of the propellants for gas generators. These stability results apply to open systems as well as to closed systems which are used in practice. The fuel used in the present invention is nitroguanidine (NIGU; NQ). Nitroguanidine is virtually non-toxic, non-hygroscopic, sparely water soluble, thermally stable, burning at low temperature and of low impact sensitivity and friction sensibility. The gas yield during burn-up is high, wherein a high portion of nitrogen gas is obtained.

[0023] A nitroguanidine which is particularly preferred according to the present invention is a nitroguanidine containing 0.1 to 0.5% nitroguanidinium hydrogen sulfate and nitroguanidinium nitrate. This kind of nitroguanidine stabilized by means of an acid will be referred to in the following as stabilized nitroguanidine. The pH value of an aqueous extract (5 g nitroguanidine per 200 ml water; 20° C.) of said stabilized nitroguanidine is in a range of from 3.5 to 4.4. Such kind of stabilized nitroguanidine is available as for example NIGU LBD SS from NIGU CHEMIE GmbH, Waldkraiburg, Germany.

[0024] Conventional NIGU has a pH value of from 4.5-7.0 (5 g nitroguanidine per 200 ml water; 20° C.).

[0025] Examples of oxidizing agents, component (B), are alkali metal- or alkaline earth metal nitrates (such as lithium nitrate, sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate, strontium nitrate or barium nitrate), alkali metal- or alkaline earth metal chlorates and -perchlorates (such as lithium-, sodium-, potassium-, magnesium-, calcium-, strontium- or barium chlorate and lithium-, sodium-, potassium-, magnesium-, calcium-, strontium- or barium perchlorate), ammonium nitrate, ammonium perchlorate, oxidizing copper compounds (such as Cu(NO₃)₂.3Cu(OH)₂ and Cu₂(OH)₃NO₃, respectively, CuCO₃ and CuO), and mixtures thereof. Potassium nitrate, potassium perchlorate, strontium nitrate, ammonium nitrate, ammonium perchlorate and Cu(NO₃)₂.3Cu(OH)₂ (copper(II)trihydroxy nitrate) are preferred. Moreover, mixtures of alkali metal- or alkaline earth metal nitrates with ammonium perchlorate are preferred and mixtures of potassium nitrate or sodium nitrate with ammonium perchlorate are in particular preferred.

[0026] Examples of stabilizers, component (C), are inorganic and organic acids. A particularly preferred inorganic acid is boric acid. Particularly preferred organic acids are citric acid, tartaric acid, cyanuric acid, terephthalic acid and fumaric acid. A further suitable stabilizer is hydrophobic SiO₂ (obtainable for example as Aerosil R812S of Degussa AG, Germany; hydrophobizing agent: hexamethylene disilazane) if stabilized NIGU is used as a fuel. Hydrophobic SiO₂ is a material which is not wetted by water, i.e. it swims on the water surface (see below, “Schriftenreihe Pigmente”, Nr. 11, pages 55ff). Preferably hydrophobic SiO₂ is present in combination with a further stabilizer.

[0027] The gaseous reaction products obtained during burn-up of the propellants for gas generators according to the present invention essentially consist of carbon dioxide, nitrogen and water vapour. Possible toxic gaseous burn-up products such as CO, NO_(x) and NH₃ are below the required maximum limits.

[0028] Nitroguanidine, component (A), is present in the propellants for gas generators according to the present invention in an amount of about 33 to about 60 percent by weight, preferably of about 40 to about 60 percent by weight and in particular of about 45 to about 55 percent by weight. The oxidizing agent, component (B), is present in an amount of about 35 to about 55 percent by weight, preferably of about 38 to about 52 percent by weight and in particular of about 40 to about 48 percent by weight. The stabilizer, component (C), is present in an amount of up to about 5 percent by weight, preferably of up to about 3 percent by weight and in particular of up to about 1.6 percent by weight and in particular is present in a range of about 0.5 to about 1.6 percent by weight.

[0029] In order to adapt to the burn-up behaviour and the gas yield as well as to improve the slag formation the propellants for gas generators according to the present invention contain further components.

[0030] The propellants for gas generators according to the present invention optionally contain as component (D) at least one burn-up stabilizer and burn-up moderator, respectively, which can also function as a slag former and slag trap, respectively. Examples include Al₂O₃, in particular highly dispersed Al₂O₃ with a BET surface (according to DIN 66131) of 100+15 m²/g (for example obtainable as Aluminiumoxid C from Degussa AG, Germany), Fe₂O₃, SiO₂, iron acetyl acetonate, mixtures thereof as well as mixtures of highly dispersed Al₂O₃ and SiO₂, for example a mixture of about 16% highly dispersed Al₂O₃ and about 84% highly dispersed SiO₂ (for example available as Aerosil COK 84 from Degussa AG, Germany) (see “Schriftenreihe Pigmente”, “Basics of Aerosil@”, No. 11, 5th edition 1993, page 38, Degussa AG).

[0031] The advantageous properties of using highly dispersed aluminum oxide in propellants for gas generators are described in DE-A-19812372 which is hereby incorported by reference. Highly dispersed aluminium oxide with a primary particle size of about 13 nm acts as a slag trap, i.e. as an internal filter in the propellant for gas generators per se. These pyrogenic oxides are prepared by means of high temperature hydrolysis (flame hydrolysis) of the gaseous metal chloride (AlCl₃) under the influence of the water obtained in the occuring hydrogen-oxygen reaction under temperature conditions which are characteristic for this kind of reaction (4 AlCl₃+6H₂+3 O₂→2 Al₂O₃+12 HCl) (see Schriftenreihe Pigmente, “Highly Dispersed Metal Oxides Obtained According To The Aerosil® Process”, No. 56, 4th edition 1989, Degussa AG).

[0032] The burn-up stabilizers and moderators, respectively, component (D), cause inter alia a linear burn-up behaviour, i.e. an exponential increase of pressure and temperature during the burn-up is prevented. Moreover, Fe₂O₃ for example can act under particular burn-up conditions as an oxygen supplier. Furthermore, these compounds can also be used as slag formers to prevent the formation of powder (dust-type) burn-up products.

[0033] Component (D) is present in the propellants for gas generators in an amount of up to about 7 percent by weight, preferably in an amount up to about 5 percent by weight and in particular in an amount of about 0.4 to about 5 percent by weight.

[0034] Highly dispersed Al₂O₃ is present in the propellants of the present invention, preferably in an amount up to 5 percent by weight, preferably in an amount of 0.5-3 percent by weight and in particular 2-3 percent by weight. Due to this low amount of Al₂O₃ a high gas yield can be guaranteed.

[0035] Furthermore, the propellants for gas generators according to the present invention may contain as component (E) at least one binder. Examples for suitable binders are cellulose compounds, polymers of one or more polymerizable olefinic unsaturated monomers, metal salts of stearic acid being insoluble in water at room temperature and graphite. Graphite is particularly preferred.

[0036] Examples of cellulose compounds are cellulose ethers, such as carboxymethylcellulose, methylcelluloseethers, in particular methylhydroxyethylcellulose, a methylhydroxyethylcellulose which can be used satisfactorily is CULMINAL®, MHEC 30000 PR supplied by the company Aqualon. Suitable polymers having binding action are polyvinylpyrrolidone, polyvinylacetate, polyvinylalcolhol and polycarbonates.

[0037] The binder, component (E), serves as desensitizing agent and as processing aid in the production of granular material or tablets (pellets) from the propellant for gas generators. It furthermore serves to reduce the hydrophilic nature of the propelling charges for gas generators.

[0038] Component (E) is present in an amount of up to about 5 percent by weight, perferably of up to about 3 percent by weight, more preferably of up to 1 percent by weight and in particular of about 0.2 to about 0.5 percent by weight.

[0039] Preferred propellants for gas generators according to the present invention comprise nitroguanidine, in particular nitroguanidine stabilized according to the present invention as a fuel (component (A)), Cu(NO₃)₂.3Cu(OH)₂, Sr(NO₃)₂, KNO₃ or a mixture of KNO₃ and NH₄ClO₄ as an oxidizing agent (component (B)), at least one stabilizer selected from the group consisting of hydrophobic SiO₂, and boric acid, citric acid, tartaric acid, cyanuric acid, terephthalic acid and fumaric acid, optionally as a mixture with hydrophobic SiO₂ (component C)), highly dispersed Al₂O₃, optionally as a mixture with iron(III) oxide as component (D) and graphite as component (E).

[0040] The object underlying the present invention is solved according to a further embodiment of the present invention by a propellant for gas generators, comprising:

[0041] (A) stabilized nitroguanidine as a fuel,

[0042] (B) Sr(NO₃)₂ or a mixture of KNO₃ or NaNO₃ and NH₄ClO₄ as oxidizing agent, and optionally

[0043] (D) a burn-up stabilizer/moderator and slag former/trap as described above and mixtures thereof and optionally

[0044] (E) a binder, as described above.

[0045] Surprisingly it has been found that a propellant for gas generators containing stabilized NIGU as a fuel and Sr(NO₃)₂ and a mixture of NaNO₃ or KNO₃ with NH₄ClO₄, respectively, as oxidizing agent shows a good or excellent long-term stability under heat aging conditions at 110° C. even in the presence of burn-up stabilizers/moderators and slag formers/traps. In such a case the addition of a stabilizer (component (C)) is not necessary for the stabilization of the propellants for gas generators.

[0046] An explanation for this excellent long-term stability is the presence of an acidic environment in the propellants for gas generators according to the present invention.

WORKING EXAMPLES

[0047] Preparation:

[0048] In general, the preparation of the propellants for gas generators and of the propelling charges for gas generators was carried out as described in the following:

[0049] A) Wet Process:

[0050] The starting materials (A), (B), (C), optionally (D) and optionally (E) have been mixed and have been milled and predensified, respectively, by means of a ball mill. The granulation of the propellant mixture for gas generators has been carried out in a vertical mixer in that about 20 percent of water has been added during stirring and at an elevated temperature of about 40° C. After a short venting and predrying, respectively, the obtained mixed mass has been grinded at room temperature through a grinding machine having a 1 mm sieve. The thus obtained granulate has been dried for about 2 hours at 80° C. in a drying ofen.

[0051] The ready-to-use granulate of the propellant for gas generators (grain size distribution 0-1 mm) has then been compressed into tablets (pellets) using a rotary pelleting machine. These pellets for the propelling charges for gas generators have been dried again at 80° C. in a drying ofen.

[0052] (B) Dry Process:

[0053] The starting materials (A), (B), (C), optionally (D) and optionally (E) are mixed in a dry state and are then compacted under pressure for example by means of a gear wheel compacting mill. Then the compacted mass is broken up to a granulate and formed into tablets by means of a rotary-pelleting machine.

[0054] The tablets and pellets, respectively, used in the gas generators and prepared in the propellants for gas generators can be prepared according to processes known in the art, for example by extruding, in rotary-pelleting (compression) machines or tabletting machines. The size of the pellets and tablets, respectively, depends on the desired burning time in each individual case.

[0055] The propellant for gas generators according to the present invention consists of non-toxic, inexpensive components which can be easily prepared and the processing of which is not problematic. The mixtures are readily ignitable. They are fast-burning and ensure high gas yields with a very low proportion of CO, NO_(x) and NH₃, which proportion is below the admissible upper limit. In particular, the propellants for gas generators according to the present invention have a very good stability under heat aging conditions of 110° C. for more than 400 hours.

[0056] Therefore, the mixtures according to the present invention are in particular suitable as gas generating agents in the various airbag systems, as well as extinguishing agents or propellants.

[0057] The following examples 1 to 24 (table II) illustrate but do not limit the present invention.

[0058] In table I comparison examples 1 to 11 are illustrated.

[0059] The superscripts of the tables have the following meaning:

[0060] 1 Aluminiumoxid C, Degussa AG

[0061] 2 Iron(III) oxide, 99.9%, ALFA Aesar—Johnson Matthey GmbH

[0062] 3 Aerosil COK 84, Degussa AG

[0063] 4 Aerosil, R 812 S, Degussa AG

[0064] Explanation of the Configuration of the Propellants T4 × 2 Tablets of 4 mm diameter and 2 mm height T3 × 1.5 Tablets of 3 mm diameter and 1.5 mm height T3 × 0.8 Tablets of 3 mm diameter and 0.8 mm height T6 × 2 Tablets of 6 mm diameter and 2 mm height

[0065] Granulate (prepared according to the wet process as described above)

[0066] The percentage are percents by weight.

[0067] GuNO₃ is an abbreviation for guanidinium nitrate and acts as an auxiliary fuel of lower energy.

[0068] In the following examples NIGU (stabilized) means nitroguanidine which is stabilized with 0.2% in total of nitroguanidinium hydrogen sulfate and nitroguanidinium nitrate. TABLE I Comparison example No. 4 6 1 2 3 a b 5 a b 7 8 9 10 11 A = NIGU (conventional) [%] 53.7 52.7 52.4 53.2 53.5 — — — — — — NIGU (stabilized) [%] — — — — — 53.5 45.0 50.8 49.8 30.0 40.0 GuNO₃ [%] — — — — — — — — — — — B = KNO₃ [%] 43.6 42.1 41.9 43.0 43.2 43.2 — — — — — Sr(NO₃)₂ [%] — — — — — — — 24.6 24.1 — — Cu(NO₃)₂*3Cu(OH)₂ [%] — — — — — — 55.0 24.6 24.1 — — CuCO₃ [%] — — — — — — — — — 70.0 — CuO [%] — — — — — — — — — — 60.0 NH₄ClO₄ [%] — — — — — — — — — — — KClO₄ [%] — — — — — — — — — — — C = Citric acid [%] — — — — — — — — — — — Tartaric acid [%] — — — — — — — — — — — SiO₂, hydrophobic⁴ [%] — — — 0.5 — — — — — — — Cyanuric acid [%] — — — — — — — — — — — Fumaric acid [%] — — — — — — — — — — — Terephthalic acid [%] — — — — — — — — — — — Boric acid [%] — — — — — — — — — — — D = Al₂O₃ ¹ [%] — 2.5 5.0 2.6 2.6 2.6 — — 2.0 — — Iron(III) oxide² [%] 2.5 2.5 0.5 0.5 0.5 0.5 — — — — — Aerosil COK 84³ [%] — — — — — — — — — — — Iron(III) acetylacetonate [%] — — — — — — — — — — — E = Graphite [%] 0.2 0.2 0.2 0.2 0.2 0.2 — — — — — Calculated Values: Gas yield (V = constant) [mol/kg] 26.0 25.5 27.9 25.9 25.9 25.9 26.6 26.6 26.1 20.1 19.2 Carbon dioxide CO₂ [Vol-%] 12.2 12.4 12.3 12.5 12.3 12.3 16.2 13.8 13.8 42.2 19.6 Nitrogen N₂ [Vol-%] 47.9 47.9 47.9 47.8 47.9 47.9 35.3 42.3 42.3 28.7 40.0 Vapour H₂O [Vol-%] 39.6 39.7 39.7 39.5 39.7 39.7 48.4 43.6 43.6 28.6 39.7 Temperature (p = 135 * 10⁵ Pa) [K] 2112 2099 2105 2119 2127 2127 2067 2406 2366 1358 1979 Thermal stability: Configuration of the propellant [mm] T4 × 2 T4 × 2 T4 × 2 T3 × 1.5 T4 × 2 T4 × 2 T3 × 1.5 T4 × 2 Granulate Granulate Granulate Granulate Granulate Aging period 400 h [%] −0.19 −1.47 −3.76 −0.49 −0.62 −1.45 −0.86 −0.84 −0.43 −0.26 −0.84 −0.26 −0.44 Aging period 1000 h [%] −0.36 — — −1.53 — — −2.74 −2.05 −3.29 −1.41 −2.52 −0.39 −0.57 Aging period 1500 h [%] — — — — — — — — — — — — — Aging period 3000 h [%] — — — — — — — — — — — — —

[0069] TABLE II Example No. 1 4 a b 2 3 a b c d e 5 6 7 8 9 10 A = NIGU (conventional) [%] — — — — 52.7 52.7 53.4 53.4 53.4 53.4 NIGU (stabilized) [%] 53.2 52.5 53.1 53.5 — — — — — — GuNO₃ [%] — — — — — — — — — — B = KNO₃ [%] 43.0 42.6 43 42.2 42.1 42.1 42.3 42.3 42.3 43.1 Sr(NO₃)₂ [%] — — — — — — — — — — Cu(NO₃)₂*3Cu(OH)₂ [%] — — — — — — — — — — CuCO₃ [%] — — — — — — — — — — CuO [%] — — — — — — — — — — NH₄ClO₄ [%] — — — — — — — — — — KClO₄ [%] — — — — — — — — — — C = Citric acid [%] — 1.0 — — 1.0 — — — — — Tartaric acid [%] — — — — — 0.1 — — — — SiO₂, hydrophobic⁴ [%] 0.5 0.6 0.6 0.6 — — 0.6 0.6 0.6 — Cyanuric acid [%] — — — — — — 0.6 — — 0.6 Fumaric acid [%] — — — — — — — 0.6 — — Terephthalic acid [%] — — — — — — — — 0.6 — Boric acid [%] — — 0.49 0.6 — — — — — — D = Al₂O₃ ¹ [%] 2.6 2.6 2.11 2.6 2.5 2.5 2.6 2.6 2.6 2.6 Iron(III) oxide² [%] 0.5 0.5 0.5 0.5 1.5 1.5 0.5 0.5 0.5 0.3 Aerosil COK 84³ [%] — — — — — — — — — — Iron(III) acetylacetonate [%] — — — — — — — — — — E = Graphite [%] 0.2 0.2 0.2 — 0.2 0.2 — — — — Calculated Values: Gas yield (V = constant) [mol/kg] 25.9 26.0 26.0 26.1 26.0 26.0 26.1 26.1 26.2 26.0 Carbon dioxide CO₂ [Vol-%] 12.5 12.9 12.6 12.3 12.57 12.6 12.6 12.7 12.5 12.1 Nitrogen N₂ [Vol-%] 47.8 46.9 47.4 47.4 47.0 47.1 47.7 47.4 47.2 47.9 Vapour H₂O [Vol-%] 39.5 38.9 39.7 40.0 38.85 39.0 39.7 39.5 38.9 39.7 Temperature (p = 135 * 10⁵ Pa) [K] 2119 2116 2116 2116 2096 2103 2116 2116 2116 2111 Thermal stability: Configuration of the propellant [mm] T3 × 1.5 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T4 × 2 T6 × 2 T6 × 2 T6 × 2 T6 × 2 Aging period 400 h [%] −0.18 −0.18 −0.06 0.02 −0.07 −0.03 −0.08 0.00 −0.03 0.00 −0.08 −0.15 −0.16 −0.10 −0.10 Aging period 1000 h [%] −0.43 −0.47 −0.09 0.00 −0.03 −0.01 −0.10 −0.02 −0.05 −0.03 −0.28 −0.22 −0.13 −0.05 −0.17 Aging period 1500 h [%] — — — — −0.08 −0.03 −0.10 −0.02 −0.07 — — — — — — Aging period 3000 h [%] — — — — −0.09 −0.05 — — — — — −0.24 −0.18 −0.10 — Example No. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 A = NIGU (conventional) [%] — — — — — — — — — — — — — — NIGU (stabilized) [%] 48.5 54.5 53.6 52.0 33.1 44.8 43.8 54.0 54.7 53.6 50.2 49.4 29.4 39.4 GuNO₃ [%] — — — — 22.0 — — — — — — — — — B = KNO₃ [%] 47.5 — 21.5 21.0 20.8 — — — — — — — — — Sr(NO₃)₂ [%] — — — — — — — 44.0 44.7 43.8 24.6 24.0 — — Cu(NO₃)₂*3Cu(OH)₂ [%] — — — — — 54.6 53.6 — — — 24.6 24.0 — — CuCO₃ [%] — — — — — — — — — — — — 70.0 — CuO [%] — — — — — — — — — — — — — 60.0 NH₄ClO₄ [%] — — 24.9 24.4 24.1 — — — — — — — — — KClO₄ [%] — 40.0 — — — — — — — — — — — — C = Citric acid [%] — — — — — — — — — — — — — — Tartaric acid [%] — — — — — — — — — — — — — — SiO₂, hydrophobic⁴ [%] — — — — — — — — — — — — — — Cyanuric acid [%] — — — — — — — — — — — — — — Fumaric acid [%] — — — — — — — — — — — — — — Terephthalic acid — — — — — — — — — — — — — — Boric acid [%] — — — — — 0.6 0.6 — 0.6 0.6 0.6 0.6 0.6 0.6 D = Al₂O₃ ¹ [%] — — — 2.6 — — 2.0 2.0 — 2.0 — 2.0 — — Iron(III) oxide² [%] — — — — — — — — — — — — — — Aerosil COK 84³ [%] 2.0 5.0 — — — — — — — — — — — — Iron(III) acetylacetonate [%] 2.0 — — — — — — — — — — — — — E = Graphite [%] — 0.5 — — — — — — — — — — — — Calculated Values: Gas yield (V = constant) [mol/kg] 25.2 29.6 34.4 33.5 35.8 26.6 26.0 25.5 26.4 25.9 26.5 26.4 19.9 19.1 Carbon dioxide CO₂ [Vol-%] 13.7 19.1 15.0 14.9 14.2 16.1 16.2 11.0 11.0 10.9 13.8 13.7 42.6 19.8 Nitrogen N₂ [Vol-%] 46.3 35.4 36.1 36.0 34.3 35.1 35.1 44.1 44.2 43.9 42.1 41.4 28.4 39.8 Vapour H₂O [Vol-%] 39.3 35.4 42.3 42.2 45.2 48.4 48.5 36.8 37.2 36.9 44.0 43.3 28.8 40.2 Temperature (p = 135 * 10⁵ Pa) [K] 2100 2797 2716 2666 2589 2174 2145 2595 2702 2665 2482 2449 1484 2116 Thermal stability: Configuration of the Propellant [mm] T3 × 0.8 Granulate T4 × 2 T4 × 2 T4 × 2 Granulate Granulate Granulate Granulate Granulate Granulate Granulate Granulate Granulate Aging period 400 h [%] −0.07 −0.05 0.11 0.00 0.11 −0.04 0.00 −0.05 −0.06 −0.06 −0.05 −0.05 −0.12 −0.07 Aging period 1000 h [%] — — 0.02 −0.19 −0.03 −0.07 −0.09 −0.21 −0.09 −0.10 0.00 −0.07 −0.12 −0.08 Aging period 1500 h [%] — — — — — −0.09 −0.20 −0.38 −0.09 −0.09 −0.20 −0.17 — — Aging period 3000 h [%] — — — — — −0.24 — — — — — — — —

[0070] The comparison examples 1 to 5 illustrate the conventional stability of propellants for gas generators on the basis of conventional nitroguanidine as a fuel.

[0071] As can be seen from comparative examples 1 to 3, the instability of propellants for gas generators increases with an increasing amount of highly dispersed Al₂O₃. According to comparative example 1 the propellant for gas generators does not contain Al₂O₃ and shows a satisfactory long-term stability over an aging period of 400 hours and 1000 hours, respectively. However, such kind of propellant for gas generators is not suitable to be used in practice since the burn-up behaviour is insufficient. Indeed, the burn-up behaviour improves with an increasing amount of Al₂O₃, however, the stability of the propellant for gas generators rapidly decreases. According to comparative example 2 there is a loss of weight after an aging period of 400 hours of 1.47% and for an amount of 5.0 percent by weight Al₂O₃ the loss of weight after an aging period of 400 hours is 3.76%. These values are inacceptable for practical applications.

[0072] Comparative example 4 shows a distinct increase of the stability by the addition of hydrophobic SiO₂. In comparison to the recipe according to comparative example 5 there is only a loss of weight of 0.62% after aging period of 400 hours in comparison to 1.45% for the recipe according to comparative example 5 for a tablet with a diameter of 4 mm and a height of 2 mm. Both recipes according to comparative example 4 as well as according to comparative example 5 contain a sufficient amount of 2.6 percent by weight Al₂O₃. However, the improvement of the stability is not sufficient in order to fulfill the requirements asked for by the automobile industry.

[0073] In the recipe according to comparative example 6 stabilized nitroguanidine has been used for the first time by the present applicant. Also in this case a distinct increase of stability can be seen in comparison to the results for the recipe according to comparative example 5. However, again, the improvement of the stability is not sufficient in order to obtain satisfactory stability results. The recipe according to comparative example 6 is not a recipe of the prior art. Based on the results obtained for comparative example 6 there has been found by the present inventors when carrying out further experiments that an acidic environment must be present in order to stabilize the propellants for gas generators based on nitroguanidine.

[0074] Comparative example 7 shows the instability of a propellant for gas generators containing stabilized nitroguanidine in the presence of Cu(NO₃)₂.3Cu(OH)₂. In comparative examples 8 and 9 a mixture of oxidizing agents comprising Sr(NO₃)₂ and Cu(NO₃)₂.3Cu(OH)₂. Al₂O₃ being present in the recipe according to comparative example 9 again causes a decrease of the stability. Finally, the stability of stabilized nitroguanidine in the presence of CuCO₃ and CuO, respectively, has been examined.

[0075] According to example 1 of the present invention (table II) a very good stabilization is obtained in a propellant for gas generators containing Al₂O₃ as component (D) by combining stabilized NIGU as a fuel and hydrophic SiO₂ as a stabilizer (see example 1 and comparative examples 4 and 6).

[0076] A further distinct improvement of the stability is obtained in the presence of a further stabilizer selected from the group of inorganic and organic acids (see examples 2 to 4).

[0077] A very good stability is also obtained for propellants for gas generators containing conventional NIGU as a fuel and Al₂O₃ as component (D) if stabilizers are used selected from the group consisting of inorganic and organic acids (see examples 5 to 10).

[0078] In examples 1 to 10 KNO₃ has been used as an oxidizing agent (component (B)) in combination with highly dispersed Al₂O₃ and iron(III) oxide as component (D).

[0079] According to example 11 a very good stability is obtained also in combination with Aerosil COK 84 and iron(III)acetylacetonate as component (D).

[0080] The recipe according to example 12 contains KClO₄ as oxidizing agent (component (B)) and Aerosil COK 84 as component (D). Also in this case a very good stability is obtained for the granulate.

[0081] Finally, examples 13 to 15 show that a stable propellant for gas generators can be obtained even in the presence of Al₂O₃ (see example 14) without the addition of a stabilizer if a mixture of KNO₃ and NH₄ClO₄ is used as an oxidizing agent in combination with stabilized NIGU as a fuel. A comparison with comparative example 5 demonstrates that such a good stability cannot be obtained with conventional nitroguanidine as a fuel and KNO₃ as oxidizing agent in the presence of Al₂O₃.

[0082] Moreover, example 15 shows the good stability of propellants for gas generators which contain besides nitroguanidine as a fuel also guanidinium nitrate (GuNO₃) as an auxiliary fuel with lower energy.

[0083] According to examples 16 and 17 a very good stability even in the presence of Al₂O₃ is obtained when Cu(NO₃)₂-3Cu(OH)₂ is used as an oxidizing agent and boric acid is used as a stabilizer.

[0084] In examples 18 to 20 Sr(NO₃)₂ has been used as an oxidizing agent. These examples show an excellent stability of stabilized nitroguanidine in the presence of Sr(NO₃)₂ and Al₂O₃. The addition of boric acid as a stabilizer even more improves the stability.

[0085] Examples 21 and 22 prove the stabilizing effect of boric acid in compositions containing Sr(NO₃)₂ and Cu(NO₃)₂.3Cu(OH)₂ as oxidizing agent (see comparative exampies 8 and 9).

[0086] Finally, the stabilizing effect of boric acid is shown in examples 23 and 24 for compositions containing CuCO₃ and CuO, respectively, as oxidizing agent (see comparative examples 10 and 11). 

1. Propellant for gas generators, comprising (A) nitroguanidine as a fuel, stabilized with 0.1 to 0.5 percent by weight nitroguanidinium hydrogen sulfate and nitroguanidinium nitrate, (B) an oxidizing agent selected from the group consisting of alkali metal and alkaline earth metal nitrates, -chlorates and -perchlorates, ammonium nitrate and -perchlorate, oxidizing copper compounds and mixtures thereof, (C) a stabilizer selected from the group consisting of inorganic and organic acids and mixtures thereof, optionally (D) a burn-up stabilizer or moderator, respectively, and slag former or slag trap, respectively, and mixtures thereof.
 2. Propellant for gas generators, comprising (A) nitroguanidine as a fuel, (B) an oxidizing agent selected from the group consisting of alkali metal and alkaline earth metal nitrates, -chlorates and -perchlorates, ammonium nitrate and -perchlorate, oxidizing copper compounds and mixtures thereof, (C) boric acid as a stabilizer, optionally (D) a burn-up stabilizer or moderator, respectively, and slag former or slag trap, respectively, and mixtures thereof.
 3. Propellant for gas generators according to claim 1 or 2, wherein component (A) is present in an amount of about 33 to about 60 percent by weight, preferably of about 40 to about 60 percent by weight and particular of about 45 to about 55 percent by weight, component (B) is present in an amount of about 35 to about 55 percent by weight, preferably of about 38 to about 52 percent by weight and in particular of about 40 to about 48 percent by weight, component (C) is present in an amount of up to about 5 percent by weight, preferably of up to about 3 percent by weight, more preferably of up to about 1.6 percent by weight and in particular of about 0.5 to 1.6 percent by weight and component (D) is present in an amount of up to about 7 percent by weight, preferably of up to about 5 percent by weight and in particular of about 0.4 to about 5 percent by weight.
 4. Propellant for gas generators according to any one of claims 1 to 3, wherein component (C) additionally comprises hydrophobic SiO₂ as a stabilizer.
 5. Propellant for gas generators according to any one of claims 1 to 4, wherein component (B) is selected from the group consisting of sodium nitrate, potassium nitrate, strontium nitrate, ammonium perchlorate, potassium chlorate, copper(II)trihydroxynitrate and mixtures thereof.
 6. Propellant for gas generators according to any one of claims 1 to 5, wherein component (B) is a mixture of potassium nitrate and ammonium perchlorate.
 7. Propellant for gas generators according to any one of claims 1 and 3 to 6, wherein component (C) is selected from the group consisting of boric acid, citric acid, tartaric acid, cyanuric acid, terepthalic acid and fumaric acid.
 8. Propellant for gas generators according to any one of claims 1 to 7, wherein as component (C) an inorganic or organic acid is present besides hydrophobic SiO₂ as stabilizer.
 9. Propellant for gas generators according to any one of claims 1 to 8, wherein component (D) is selected from the group consisting of Al₂O₃, Fe₂O₃, SiO₂, iron acetylacetonate, mixtures thereof and mixtures of highly dispersed Al₂O₃ with highly dispersed SiO₂.
 10. Propellant for gas generators according to claim 9, wherein Al₂O₃ is highly dispersed Al₂O₃.
 11. Propellant for gas generators according to claim 9, wherein component (D) is a mixture of about 16 percent by weight highly dispersed Al₂O₃ and about 84 percent by weight highly dispered SiO₂.
 12. Propellant for gas generators according to any one of claims 1 to 11 further comprising as component (E) at least one binder.
 13. Propellant for gas generators according to claim 12, wherein component (E) is selected from cellulose compounds, polymers of one or more polymerizable olefinic unsaturated monomers, a metal salt of stearic acid being insoluble in water at room temperature and graphite.
 14. Propellant for gas generators according to claim 12 or 13, wherein component (E) is present in amount of up to about 5 percent by weight, preferably of up to about 3 percent by weight, more preferably of up to about 1 percent by weight and in particular of about 0.2 to about 0.5 percent by weight.
 15. Propellant for gas generators according to any one of claims 1 to 14, comprising (A) nitroguanidine stabilized with 0.1 to 0.5 percent by weight nitroguanidinium hydrogen sulfate and nitroguanidinium nitrate as a fuel, (B) KNO₃ or a mixture of KNO₃ and NH₄ClO₄ as oxidizing agent, (C) a stabilizer selected from the group consisting of hydrophobic SiO₂, boric acid, citric acid, tartaric acid, cyanuric acid, terephthalic acid, fumaric acid and mixtures thereof, (D) highly dispersed Al₂O₃, optionally as a mixture with Fe₂O₃ and (E) graphite.
 16. Propellant for gas generators comprising (A) nitroguanidine stabilized with 0.1 to 0.5% nitroguanidinium hydrogen sulfate and nitroguaninidinium nitrate as a fuel, (B) Sr(NO₃)₂ or a mixture of KNO₃ or NaNO₃ with NH₄ClO₄ as oxidizing agent, an optionally (D) at least one burn-up stabilizer/moderator and slag former/slag trap.
 17. Propellant for gas generators according to claim 16, wherein component (D) is selected from the group consisting of Al₂O₃, highly dispersed Al₂O₃, Fe₂O₃, SiO₂, iron acetyl acetonate, mixtures thereof and a mixture of highly dispersed Al₂O₃ with highly dispersed SiO₂.
 18. Propellant for gas generators according to claim 16 or 17, wherein component (A) is present in an amount of about 33 to about 60 percent by weight, preferably of about 40 to about 60 percent by weight and in particular of about 45 to about 55 percent by weight, component (B) is present in an amount of about 35 to about 55 percent by weight, preferably of about 38 to about 52 percent by weight and in particular of about 40 to about 48 percent by weight and component (D) is present in an amount of up to about 7 percent by weight, preferably of about 5 percent by weight and in particular of about 0.4 to about 5 percent by weight.
 19. Propellant for gas generators according to any one of claims 16 to 18, further comprising as component (E) at least one binder.
 20. Propellant for gas generators according to claim 19, wherein the binder is selected from the group consisting of cellulose compounds, polymers of one or more polymerizable olefinic unsaturated monomers, a metal salt of stearic acid being insoluble in water at room temperature and graphite.
 21. Use of a propellant for gas generators according to any one of claims 1 to 20 as gas generating agent in airbags, as extinguishing agent or as propellant. 